1. Field of the Invention
The present invention relates to an optical amplification apparatus for light amplification, an optical communication apparatus, and an optical communication method.
2. Description of the Related Art
With the increase in communication traffic in recent years, demand for optical communication apparatuses have increased. Besides optical repeaters introduced into main networks, recently optical communication apparatuses have been actively introduced into local networks, and optical networks have also been established in subscriber loop systems. Such optical communication systems play important roles in worldwide information networks.
In optical communication systems, optical amplification/repeater systems that include wavelength-multiplexed optical amplifiers (erbium doped fiber amplifiers (EDFA)) on each transmission path are used to realize long-distance transmission having greater capacities at a lower cost and with higher reliability. When repeater associated loss in optical amplification/repeater systems is large due to increased transmission path length, for example, the power of signal components included in an optical signal input into an optical amplifier decreases, resulting in deterioration of signal/noise (SN) ratios and potentially leading to degradation of transmission characteristics.
As a countermeasure technique, distributed Raman amplification (DRA) is used to input pump light into a transmission path so as to amplify, by utilizing the Raman effect, optical signals passing through the transmission path, thereby increasing the power of the signal components. As a result, SN ratios increase and transmission characteristics improve; hence, distributed Raman amplifiers have been in practical use as an effective technique.
In optical communication systems, the longer the distance between repeater points is, the greater optical loss in a transmission path becomes. Optical loss in a typical transmission path is on the order of 0.2 dB/km, and transmission path loss increases commensurately with the distance between repeater points. When various optical elements are arranged on a transmission path, further optical signal loss occurs as a result of the transmission loss associated with each of the optical elements. The greater the repeater associated loss, the smaller the power of the optical signal becomes.
In optical communication systems, optical amplification techniques (EDFA, Raman amplification, or the like) are typically used. The larger the gains by the optical amplification techniques, the more noise components (spontaneous emission light) there are. Thus, when optical amplification techniques having larger gains are employed in transmission paths exhibiting larger repeater losses, the ratio of signal component power relative to the power of noise components included in optical signals decreases. When optical signals are provided by wavelength-multiplexed light, smaller numbers of signal wavelengths lead to smaller ratios of signal component power relative to the power of noise components included in the optical signals.
Power controlling apparatuses, used in optical communication systems, not only simply amplify optical signals but further control the power of the optical signals to be constant. A power controlling apparatus typically uses a branching coupler and a photo diode (PD) to monitor a total power (signal component+noise component) of an optical signal and to control, per channel, the power of a signal component based on information received from a monitoring system or the like concerning the number of signal wavelengths.
The purpose of a power controlling apparatus is to control the power of signal components included in an optical signal. However, when power control is performed based on the total power of a monitored optical signal, if the power of the noise components is large relative to the power of signal components, the control deteriorates (see, for example, Japanese Patent Application Laid-Open Publication No. 2000-232433). Failure to control the power of the signal components at a normal level, may potentially lead to a problem such as deterioration of transmission characteristics.
For example, increased power of the signal components included in an optical signal leads to deterioration of the signal components due to non-linear characteristics of transmission paths, thereby increasing the possibility of reception errors occurring. On the contrary, decreased power of signal components included in an optical signal leads to the occurrence of deteriorated transmission waveforms due to the effect of SN ratios, thereby increasing the possibility of reception errors occurring. As such, Japanese Patent Application Laid-Open Publication No. 2006-189465 discloses a power controlling apparatus that is configured to estimate the power of a noise component included in an optical signal and to subtract the estimated power from a total power of the optical signal to thereby calculate the power of a signal component included in the optical signal.
However, since power of the noise components occurring in the optical signal varies in a complicated manner depending on conditions of transmission paths, a problem arises in that accurate calculation of the power of the noise components is difficult. For example, gain characteristics in Raman amplification vary depending on fluctuation of design parameters or optical characteristics of transmission paths. Variations of gain characteristics in Raman amplification further lead to variation in the power of the noise components caused by Raman amplification.
For example, optical characteristics of transmission paths vary depending on: contamination at a connecting portion of an optical connector that connects optical fibers with each other; optical loss such as that due to bending loss of an optical fiber; manufacturing tolerance (e.g., loss coefficient and effective cross-sectional area) affecting characteristics of the transmission path fiber itself; variations in loss due to fusion bonded portions in transmission path fibers; aged deterioration; and ambient air temperature.
The technique according to Japanese Patent Application Laid-Open Publication No. 2006-189465 is configured to monitor a power of pump light at a Raman amplifier, to calculate Raman gain based on the monitored result, and to calculate the power of noise components from the calculated gain. However, as pumping efficiencies of Raman amplification vary depending on the type of transmission path, in practice, accurate estimation of the pump light power to be input into a transmission path is difficult. Further, when loss at the output side of a transmission paths increases as a result of, for example, application of a certain load to a fiber or connector, even when there is no variation in the monitored pump light, the power of pump light actually input to the transmission paths decreases, thereby resulting in decreased gain and optical noise.
In these situations (such as variations in transmission path loss, pump light power monitoring errors, temperature characteristics, and aged deterioration), inherently proportional relationships between pump light power and gain are not constant, thereby making accurate estimation of optical noise power difficult. Thus, characteristics of noise component power with respect to pump light power are not constant. As a result, with the technique disclosed in Japanese Patent Application Laid-Open Publication No. 2006-189465, a problem arises in that the power of noise components cannot be calculated with high accuracy.
It is conceivable to exclude, from being subject to system support, optical signals having fewer channels where the ratio of the power of noise components relative to the power of the signal components becomes larger. Although this enables a decrease of the ratio of the power of the noise components relative to the power of the signal components, optical signals subject to system support are limited, thereby significantly reducing convenience.
It is further conceivable to prepare a database of the noise component power that occurs according to system conditions, respectively. However, the noise component power occurring in Raman amplification varies according to system conditions (gains of Raman amplification, repeater loss, transmission path type, loss coefficients of transmission path fibers, effective cross-sectional areas of transmission paths, transmission path lengths, and the like).
As a result, a problem arises in that an extensive database must be maintained to realize power control with greater accuracy. Further, maintenance of such an extensive database causes another problem in that the process of selecting appropriate values from the database takes time, making retrieval of the power of the noise components in real-time difficult.